In recent years, from social demands for low-fuel consumption and low exhaust emission, there has been an increased attention to an electric vehicle or a hybrid vehicle equipped with an alternate-current (AC) motor to run. For example, in a hybrid vehicle, an AC motor is connected to a direct-current (DC) power source such as a rechargeable battery unit through a power converter such as an inverter. The inverter converts a DC voltage supplied from the DC power source to an AC voltage and drives the AC motor by the AC voltage.
US 2008/0079385 corresponding to JP-2008-86139 discloses a technique for reducing the number of current sensors used in a control apparatus for an AC motor mounted on an electric vehicle or a hybrid vehicle, thereby reducing the size of the apparatus near output terminals of the inverter and the cost of a control system of the AC motor. In the technique disclosed in US 2008/0079385, a current sensor for detecting a phase current is provided to one phase of the AC motor.
JP-4942425 discloses that two current sensors are separately provided to two phases of an AC motor. In a technique disclosed in JP-4942425, the motor is controlled based on detection values of two current sensors in low speed conditions and controlled based on a detection value of one current sensor in the other conditions to prevent a torque ripple caused by a difference in gain between the current sensors. The control using the detection values of two current sensors is hereinafter referred to as the “two-phase control”, and the control using the detection value of one current sensor is hereinafter referred to as the “one-phase control”.